1. Field of the Invention
The present invention relates to a power device, and more particularly, to a MOS control diode having two electrodes and a method for manufacturing the same.
2. Description of Related Art
A semiconductor device can be used not only for processing information or signals but also controlling the currents or power of an electrical circuit or an electronic circuit. The device used for controlling the power is called a power device in order to distinguish it from other signal processing devices. The power device has a vertical structure in which an area through which a current flows is wide and a withstand voltage is easily amplified in order to control large power. Such a vertical structure is completely different from the signal processing device, which conventionally has a lateral structure.
As electronic equipments require high function and high degree of efficiency, a miniaturizable switching source of electric power is required. Also, a diode having little switching loss has been generally required as the switching source of electric power. Impurities such as Au or Pt are doped into PN junction or particles such as electrons and protons are irradiated into the silicon substrate adjacent the PN junction to provide recombination centers. By controlling minority carrier lifetime, a reverse recovery time (Trr) characteristic of the current in a PN junction diode is improved, and thus high-speed switching of the electric power switching source is realized. However, the above doping or irradiating method has shortcomings in that manufacturing processes are difficult, expensive and increase reverse leakage current.
If a Schottky diode using a conventional Schottky barrier is used as a switching source of electric power, high-speed switching can be achieved without additional processes for increasing the Trr. A minority carrier storage effect does not exist in the Schottky diode operating by a majority carrier. However, the Schottky diode has shortcomings in that a reverse voltage is low and a reverse current is large.
Therefore, recently, there has been an attempt to realize the high speed switching using a metal oxide semiconductor field effect transistor (MOSFET) which is the majority carrier device. Research on this device is described in pages 336 through 343 of Power Semiconductor Devices published in 1996 by PWS Publishing company, a division of Thomson Publishing Inc. and written by B. JAYANT BALIGA.
In the case of constructing the switching device by three terminals comprised of a source, a drain, and a gate electrode like the power MOSFET, high speed switching can be realized. However, a desirable degree of integration of the switching device having three terminals is hard to be realized and a power circuit adopting such MOSFET switching device becomes complicated.
The present invention is a MOS control diode in which a switching operation is very fast and a reverse leakage current is lowered by contacting a gate electrode of a MOSFET with drain area, thereby converting a MOSFET which is a majority carrier device to MOS diode having two terminals.
Another aspect of the present invention is a method for manufacturing the MOS control diode.
A MOS control diode according to a first embodiment of the present invention includes a semiconductor substrate having a first conductive type as a drain area. A gate oxide film or layer is formed on a surface of the semiconductor substrate with a discontinuous area in the middle of the oxide layer, thus giving the oxide layer a disconnected shape. A gate electrode is connected to the semiconductor substrate through the discontinuous area of the gate oxide film. A base well area having a second conductive type is formed around the semiconductor substrate below the gate oxide film having a disconnected shape, and a source well area having a first conductive type is formed in the base well area.
The MOS control diode according to the first embodiment of the present invention preferably further includes an insulating film formed to cover the entire exposed surface of the gate electrode, a conductive source electrode formed on the source well area, connected to the source area, and covering the entire semiconductor substrate, and a conductive drain electrode formed on the backside of the semiconductor substrate. Preferably, the gate electrode is a composite layer of a polysilicon layer and a metal layer. The first and second conductive types can be N and P types, respectively.
A trench type MOS control diode according to a second embodiment of the present invention includes a semiconductor substrate having a first conductive type as a drain area, a base well area having a second conductive type preferably formed by performing a first ion implantation in the semiconductor substrate, and a source well area having a first conductive type formed in the second conductive base well area preferably by a second ion implantation. A gate electrode insulated from the base well area and the source well area and burying the trench formed by etching the semiconductor substrate. An insulating film is formed on the semiconductor substrate to cover the entire top side of the gate electrode and to expose a part of the source well.
Preferably, the trench type MOS control diode further includes a source electrode formed on the exposed source well and the insulating film so as to cover the entire semiconductor substrate.
According to a preferred embodiment of the present invention, the trench type MOS control diode further includes gate oxide films formed on the sidewall of the gate electrode and a drain electrode formed on a backside of the semiconductor substrate.
Preferably, the trench is formed by etching the semiconductor substrate to a depth which is deeper than the base well area is formed. Also, the gate electrode is preferably formed of a composite layer consisting of a polysilicon layer formed on the sidewall of the trench to have a certain thickness and a metal layer which completely buries the trench.
In a method for manufacturing a MOS control diode according to the first embodiment of the present invention, a gate oxide film is formed on a semiconductor substrate having a first conductive type to have a discontinuous area in the middle of the layer, thus having a disconnected shape. A gate electrode contacting the semiconductor substrate through the discontinuous area is formed on the gate oxide film. A base well area having a second conductive type is formed on the semiconductor substrate by performing a first ion implantation using the gate electrode as a mask. A source well area having a first conductive type is formed in the base well area by performing a second ion implantation. An insulating film enclosing the entire gate electrode is formed. A source electrode connected to the source well area and covering the top side of the semiconductor substrate is formed on the resultant structure. Fabrication of the MOS control diode according to the first embodiment of the present invention is completed by forming a drain electrode on backside of the semiconductor substrate.
In a method for fabricating a trench type MOS control diode according to a second embodiment of the present invention, a base well area having a second conductive type is formed by performing a first ion implantation to a semiconductor substrate having a first conductive type. A source well area having a first conductive type is formed in the base well area by performing a second ion implantation. A trench is formed so that a part of the semiconductor substrate is etched to be deeper than the base well area in the semiconductor substrate. Gate oxide films are formed on the sidewalls of the trench. Gate electrodes for burying the trench are formed.
According to a preferred embodiment of the present invention, the trench and the gate electrode including the gate oxide film are preferably plural.
In a method for forming the gate electrode, a polysilicon layer is formed on the sidewall and the bottom of the trench to have a certain thickness. A conductive material for completely burying the trench is stacked on the polysilicon layer. The conductive material is etched back so that the semiconductor substrate is exposed. Also, it is preferable that an insulating film is formed on the semiconductor substrate on which the gate electrodes are formed so as to expose a part of the source well area and to cover the entire top side of the gate electrode, and then a source electrode is formed on the exposed source well and the insulating film. It is preferable that a drain electrode is additionally formed on the backside of the semiconductor substrate.
The foregoing and other objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention which proceeds with reference to the accompanying drawings.